Method and apparatus for producing pressurized oxygen by low-temperature separation of air

ABSTRACT

The invention relates to a method and apparatus for producing pressurized oxygen by low-temperature separation of air in a distillation column system having a low-pressure column and a high-pressure column. All of the feed air is compressed to a first air pressure, at least 4 bar above the operating pressure of the high-pressure column. A first partial stream of the feed air, compressed to the first air pressure, is further compressed in a secondary compressor to a second higher air pressure, before being (pseudo-)liquefied and is introduced into the distillation system. A second partial stream of the feed air, compressed to the first air pressure, is cooled and depressurized in an expander, which drives the secondary compressor. A return stream is branched off from the (pseudo-)liquefied first partial stream, depressurized, heated and then introduced to the first partial stream upstream from the secondary compressor.

SUMMARY OF THE INVENTION

The invention relates in general to a method for producing pressurized oxygen by low-temperature separation of air in a distillation column system for nitrogen-oxygen separation, wherein the distillation column system comprises at least a low-pressure column and a high-pressure column.

Methods and devices for low-temperature separation of air are known from, for example, Hausen/Linde, Tieftemperaturtechnik [Low-Temperature Technology], 2^(nd) Edition 1985, Chapter 4 (pages 281 to 337).

The distillation column system of the invention can be a two-column system (for example, a conventional Linde double-column system) or the system can be a three- or multiple-column system. In addition to the columns for nitrogen-oxygen separation, the distillation column system can have additional devices for recovering extremely pure products and/or other air components, in particular for recovery of noble gases, for example argon recovery and/or krypton-xenon recovery.

In the process, an oxygen product stream that is pressurized in the liquid state is evaporated by heat exchange with a heat transfer medium and ultimately recovered as a pressurized gaseous product. This method is also referred to as internal compression. It is used to recover pressurized oxygen. For the case of a supercritical pressure, no phase transition takes place in the actual sense; the product stream is then “pseudo-evaporated.”

As a result of the heat exchange with the (pseudo-)evaporating product stream, a heat transfer medium that is under high pressure is liquefied (or pseudo-liquefied, when it is below supercritical pressure). The heat transfer medium is frequently formed by a portion of the air, in this case by a “second partial stream” of the compressed feed air.

Internal compression methods are known from, for example, DE 830805, DE 901542 (=U.S. Pat. No. 2,712,738/U.S. Pat. No. 2,784,572), DE 952908, DE 1103363 (=U.S. Pat. No. 3,083,544), DE 1112997 (=U.S. Pat. No. 3,214,925), DE 1124529, DE 1117616 (=U.S. Pat. No. 3,280,574), DE 1226616 (=U.S. Pat. No. 3,216,206), DE 1229561 (=U.S. Pat. No. 3,222,878), DE 1199293, DE 1187248 (=U.S. Pat. No. 3,714,96), DE 1235347, DE 1258882 (=U.S. Pat. No. 3,426,543), DE 1263037 (=U.S. Pat. No. 3,401,531), DE 1501722 (=U.S. Pat. No. 3,416,323), DE 1501723 (=U.S. Pat. No. 3,500,651), DE 253132 (=U.S. Pat. No. 4,279,631), DE 2646690, EP 93448 B1 (=U.S. Pat. No. 4,555,256), EP 384483 B1 (=U.S. Pat. No. 5,036,672), EP 505812 B1 (=U.S. Pat. No. 5,263,328), EP 716280 B1 (=U.S. Pat. No. 5,644,934), EP 842385 B1 (=U.S. Pat. No. 5,953,937), EP 758733 B1 (=U.S. Pat. No. 5,845,517), EP 895045 B1 (=U.S. Pat. No. 6,038,885), DE 19803437 A1, EP 949471 B1 (=U.S. Pat. No. 6,185,960 B1), EP 955509 A1 (=U.S. Pat. No. 6,196,022 B1), EP 1031804 A1 (=U.S. Pat. No. 6,314,755), DE 19909744 A1, EP 1067345 A1 (=U.S. Pat. No. 336,345), EP 1074805 A1 (=U.S. Pat. No. 6,332,337), DE 19954593 A1, EP 1134525 A1 (=U.S. Pat. No. 6,477,860), DE 10013073 A1, EP 1139046 A1, EP 1146301 A1, EP 1150082 A1, EP 1213552 A1, DE 10115258 A1, EP 1284404 A1 (=US 2003051504 A1), EP 1308680 A1 (=U.S. Pat. No. 6,612,129 B2), DE 10213212 A1, DE 10213211 A1, EP 1357342 A1 or DE 10238282 A1, DE 10302389 A1, DE 10334559 A1, DE 10334560 A1, DE 10332863 A1, EP 1544559 A1, EP 1585926 A1, DE 102005029274 A1, EP 1666824 A1, EP 1672301 A1, DE 102005028012 A1, WO 2007033838 A1, WO 2007104449 A1, EP 1845324 A1, DE 102006032731 A1, EP 1892490 A1, DE 102007014643 A1, A1, EP 2015012 A2, EP 2015013 A2, EP 2026024 A1, WO 2009095188 A2, or DE 102008016355 A1.

In addition to the pressurized oxygen product, other typical air-separation products can be obtained in the form of internally-compressed liquid or gaseous streams.

The “main heat exchanger system” serves to cool the feed air in indirect heat exchange with backflows from the distillation column system. It can be formed from one or more parallel- and/or serially-connected heat-exchanger sections, for example from one or more plate heat-exchanger blocks.

The method of the invention is part of the class of high-pressure methods in which all of the air is compressed in a main air compressor to significantly above the high-pressure column pressure. In particular, the system works with a single externally-driven machine, i.e. said main air compressor.

The configuration in which the compressed, cooled and purified air is divided into two streams—a turbine flow (“second partial stream”) and a choke feed (“first partial stream”) is especially efficient. The turbine flow is cooled in the heat exchanger and then depressurized in a turbine to, for example, the pressure of the high-pressure column and directed into the latter. The choke feed is further compressed in a booster stage (“secondary compressor”), driven by the turbine, run through the heat exchanger and in this case cooled, and is then directed into the rectification part.

A method of the above-mentioned type is known from U.S. Pat. No. 5,329,776.

It is often technically impossible to implement such a method, especially when the choke feed is relatively small (less than ⅓ of all of the air) and thus the turbine flow is correspondingly large (correspondingly larger than ⅔ of all of the air). Such a turbine namely cannot be easily built since the amounts of choke and turbine are too different and the pressure ratio in the booster stage is too large. Even if the turbine can be built, the disproportion between the booster and turbine sides results in efficiency compromises, which impair the overall efficiency of the unit.

An aspect of the invention is therefore to provide a method of the above-mentioned type and a corresponding apparatus, which are especially advantageous economically, in particular also in the case where a relatively small choke feed (“first partial stream”) is used.

Upon further study of the specification and appended claims, other aspects and advantages of the invention will become apparent.

Therefore, in accordance with the invention, there is provided a method for producing pressurized oxygen by low-temperature separation of air in a distillation column system for nitrogen-oxygen separation, comprising a low-pressure column and a high-pressure column, wherein:

all of the feed air is compressed to a first air pressure, which is at least 4 bar over the operating pressure of the high-pressure column,

at least a portion of the compressed feed air is cooled in a main heat exchanger,

a first partial stream of the feed air, that is compressed to the first air pressure, is further compressed in a secondary compressor to a second air pressure that is higher than the first air pressure,

the further-compressed first partial stream is liquefied or pseudo-liquefied in a main heat exchanger and then is introduced at least in part into the distillation column system for nitrogen-oxygen separation,

a second partial stream of the feed air, compressed to the first air pressure, is cooled in the main heat exchanger to an intermediate temperature and then is actively depressurized in an expander, whereby the expander drives the secondary compressor,

the expanded second partial stream is introduced into the distillation column system for nitrogen-oxygen separation,

an oxygen product stream is drawn off in liquid form from the distillation column system for nitrogen-oxygen separation, and is brought into the liquid state at an elevated pressure,

the oxygen product stream that is pressurized in liquid form is evaporated or pseudo-evaporated and heated under elevated pressure in the main heat exchanger and finally recovered as a pressurized oxygen product,

a return stream is branched off from the (pseudo-)liquefied first partial stream,

the return stream is depressurized in a choke (throttle) valve and heated in the main heat exchanger, and

the heated return stream is fed to the first partial stream at a point upstream of the secondary compressor.

The return stream, which is formed by a portion of the first partial stream of the air (of the choke feed) that is run through the secondary compressor, flows in a circuit through the secondary compressor. Thus, the amount of air to be compressed in the secondary compressor is increased. The return stream is preferably removed at the exit from the main heat exchanger (on the cold end), depressurized in a choke valve to a pressure that is somewhat higher than the pressure before entry into the secondary compressor. The expanded return stream is directed through the main heat exchanger and further mixed with the first partial stream before its entry into the secondary compressor. This recirculation of the choke feed results in the amount of gas that is to be compressed in the booster stage (the secondary compressor) being larger, and thus the pressure ratio is smaller, and the amounts of booster and turbine are better matched to one another. Thus, a booster turbine that can be built with conventional means and that is especially efficient can be used.

The return stream can be heated up to the temperature of the hot end of the main heat exchanger or can be heated up to an intermediate temperature.

The introduction of the return stream into the first partial stream upstream from the secondary compressor can also in principle be done by having the return stream be introduced into the overall air stream before it is split; the “return into the first partial stream” is then performed upstream of the point where the first partial stream is branched off from the second partial stream. Preferably, however, the return stream is introduced into the first partial stream at a point immediately before the secondary compressor.

At first, it seems illogical that the choke depressurization and re-compression of a stream (the return stream) will make the method more economically advantageous. Within the scope of the invention, however, it has turned out, surprisingly enough, that the efficiency can be increased by this measure. This can be explained by the surprisingly efficient depressurization in the low or supercritical state, and as a result, the heating of the return stream, surprisingly enough, contributes significantly to improving the heat exchange process in the main heat exchanger.

Preferably, the system has only a single externally-driven compressor, the main air compressor, and only a single expander (turbine). Despite this comparatively simple design, the system is very energy-efficient owing to the approach according to the invention.

In the invention, all of the air is compressed to a first air pressure, which is, for example, 10 to 30 bar, and preferably between 10 and 20 bar.

Other configurations of the method according to the invention are described in the dependent process claims.

In addition, the invention relates to an apparatus for producing pressurized oxygen by low-temperature separation comprising:

-   -   a distillation column system for nitrogen-oxygen separation,         comprising a low-pressure column and a high-pressure column,     -   a main compressor for compressing all of the feed air to a first         air pressure, which is at least 4 bar above the operating         pressure of the high-pressure column,     -   a main heat exchanger for cooling at least a portion of the         compressed feed air,     -   a secondary compressor for further compressing a first partial         stream of the feed air which is at the first air pressure to a         second air pressure, which is higher than the first air         pressure,     -   means for liquefying or pseudo-liquefying the further-compressed         first partial stream in the main heat exchanger and means for         introducing at least a portion of the (pseudo-)liquefied first         partial stream into the distillation column system for         nitrogen-oxygen separation,     -   means for cooling a second partial stream of the feed air, that         is compressed to the first air pressure, in the main heat         exchanger to an intermediate temperature,     -   an expander for active depressurization of the cooled second         partial stream, wherein the expander is mechanically coupled to         the secondary compressor,     -   means for introducing the expanded second partial stream into         the distillation column system for nitrogen-oxygen separation,     -   means for withdrawing an oxygen product stream in liquid state         from the distillation column system for nitrogen-oxygen         separation,     -   means for increasing the pressure of the oxygen product stream         in the liquid state to an elevated pressure, and     -   means for evaporation or pseudo-evaporation and heating of the         oxygen product stream, pressurized in liquid state, under         elevated pressure in the main heat exchanger, and means for         recovery of the heated oxygen product stream as a pressurized         oxygen product,     -   means for branching off a return stream from the         (pseudo-)liquefied first partial stream,     -   a choke valve for depressurizing the return stream,     -   means for heating the expanded return stream in the main heat         exchanger, and     -   means for supplying the heated return stream to the first         partial stream to a point upstream from the secondary         compressor.

The device according to the invention can be supplemented by device features, which correspond to the features of the dependent process claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention as well as further advantages, features and examples of the present invention are explained in more detail by the following descriptions of embodiments based on the Figures, wherein:

FIG. 1 shows a first embodiment of the invention with a single turbine and a hot secondary compressor,

FIG. 2 shows a second embodiment, in which the secondary compressor is designed as a cold compressor,

FIG. 3 shows a third embodiment with two turbines,

FIG. 4 shows a variant with a branching-off of the return stream upstream from the cold end of the main heat exchanger, and

FIG. 5 shows another variant with secondary compression of all of the air.

In the method and apparatus of FIG. 1, atmospheric air 1 is suctioned in via a filter 2 by a main air compressor 3 and compressed there to a first air pressure of approximately 18 bar. The main air compressor 3 preferably has multiple stages with intermediate cooling. All of the compressed air is then cooled in a precooling system 4, and purified in a purification system 5. The purified feed air 6 under the first air pressure is divided into a first partial stream 7 and a second partial stream 8. In this embodiment, there are no additional partial air streams that are guided into the separation; the feed air is completely divided into the first and the second partial streams 7, 8. (This does not prevent using smaller portions of all of the air 6 for other purposes, for example as instrument air.) In this embodiment, approximately 45% (molar percent) of all of the air 6 forms the first partial stream 7, and the remainder forms the second partial stream 8. In general, the proportion of the feed air 6 that forms the first partial stream 7 can be, for example, generally 30%-70%, preferably 40%-60%.

The first partial stream 7 is fed via line 9 to a secondary compressor 10 with a secondary condenser 11 and is further compressed there to a second air pressure of approximately 28 bar. The secondary compressor 10 is designed as a single stage stage. The further-compressed first partial stream 12 is introduced into a main heat exchanger 13 at the hot end (which is, for example, at about ambient temperature), cooled therein, liquefied, and finally withdrawn from the cold end of the main heat exchanger 13 via line 14. The majority of the liquefied first partial stream is introduced via line 15 and choke (throttle) valve 16 into the distillation column system for nitrogen-oxygen separation. The distillation column system has a high-pressure column 17, a low-pressure column 18, and a main condenser 19, which is designed as a condenser-evaporator. The operating pressures (in each case at the top) are 5 to 6 bar in the high-pressure column and 1.2 to 1.6 bar in the low-pressure column. A portion of the liquid air that is introduced into the high-pressure column 17 is drawn off again via line 20, cooled in a subcooling countercurrent device 21, and fed via choke (throttle) valve 22 into the low-pressure column 18.

The second partial stream 8 of the feed air is cooled only to an intermediate temperature in the main heat exchanger 13. The cooled second partial stream 23 is introduced into an expander, which is formed here by a turbine 24. In the turbine 24, the second air stream is actively depressurized to a pressure slightly above (e.g., 0.01-0.5 bar) the operating pressure of the high-pressure column pressure. The expanded second partial stream 25 is introduced in a completely gaseous or an essentially completely gaseous state into the high-pressure column at a point directly over the bottom. The expander 24 is coupled directly mechanically to the secondary compressor 10; in particular, the secondary compressor and the turbine are seated on a common shaft.

The bottom liquid 26 of the high-pressure column 17 is cooled in the subcooling countercurrent device 21 and introduced via an argon production unit 30 and the lines 27, 28 and 29 into the low-pressure column 18. The argon production unit 30 has a divided crude argon column and a pure argon column, is fed by an argon transfer fraction 31, and yields a liquid pure argon product (LAR) 32. Moreover, it works according to known principles.

At least a portion 34 of the top nitrogen 33 of the high-pressure column 17 is condensed in the main condenser 19. At least a portion 36 of the liquid nitrogen that is obtained in this way is sent as reflux to the high-pressure column 17. A second portion 37 is cooled in a subcooling countercurrent device 21 and fed via choke (throttle) valve 38 in the top of the low-pressure column 18. At least one portion thereof is used as liquid reflux in the low-pressure column 18.

From the bottom of the low-pressure column 18 (whose lower section here simultaneously forms the evaporation chamber of the main condenser 19), an oxygen-product stream 39 is drawn off in liquid form and brought to an elevated pressure 40 (e.g., via a pump) of approximately 30 bar in the liquid state. The high-pressure oxygen 41 is evaporated in the main heat exchanger 13 and heated to approximately ambient temperature. Via line 42, it is finally recovered as internally-compressed pressurized oxygen product (GOX-IC).

Other possible products of the distillation column system for nitrogen-oxygen separation are:

-   -   Liquid oxygen 43 (LOX)—Part of the bottom oxygen 39 of the         low-pressure column 18.     -   Liquid nitrogen 44 (LIN)—Part of the second part 37, 38 of the         liquid nitrogen 35 from the main condenser 19.     -   Internally-compressed pressurized gaseous nitrogen 47         (GAN-IC)—Third part 45 of the liquid nitrogen 35 from the main         condenser 19, brought to an elevated pressure 46 (e.g., via a         pump).     -   Pressurized gaseous nitrogen 49 (PGAN) directly from the         high-pressure column 17 as a part 48 of the top gaseous nitrogen         33 of the high-pressure column 17.     -   Low pressure gaseous nitrogen 50, 61, 62 (GAN) from the top of         the low-pressure column 18.     -   Nitrogen-rich residual gas 51, 52, 53, 54 from an intermediate         point on the low-pressure column 18.

A portion 60 of the hot nitrogen-rich residual gas 54 can be used as regeneration gas (reggas) in the purification system 5.

In addition, the liquid argon 32 from the argon portion 30 can also be internally compressed in a pump 55 and can be recovered as pressurized gaseous product (GAR-IC) after (pseudo-)evaporation via line 56.

According to the invention, a return stream 57 is branched off from the liquefied partial stream 14, downstream from the cold end of the main heat exchanger 13. The return stream 57, expanded under cold conditions in a choke (throttle) valve 58 to a pressure of approximately more than 18 bar, is fed back into the cold end of the main heat exchanger 13 and heated there to an intermediate temperature of 260 K. The heated return stream 59 is finally fed to the first partial stream 7 upstream from the secondary compressor 10.

The secondary compressor 10 in FIG. 1 is operated under hot conditions, i.e., with a starting temperature that is no more than 20 K below the ambient temperature.

FIG. 2 is distinguished from FIG. 1 in that the secondary compressor 110 is operated as a cold compressor, i.e., with a starting temperature that is more than 20K below ambient temperature, in this example at 210 K. Correspondingly, the return stream 159 is also removed at a lower intermediate temperature of approximately 205 K from the main heat exchanger 13 and is combined with the first partial stream 107 only downstream from the cooling of the first partial stream 107. The division of the air feed stream into the first partial stream 107 and the second partial stream 123 takes place here within the main heat exchanger 13. The further-compressed first partial stream 112 is sent to the main heat exchanger 13 at an intermediate temperature.

To be able to discharge heat to the environment, the expander 124 is not only mechanically coupled to the cold secondary compressor 110. But, in addition, an oil brake 161 sits on the common shaft. (As an alternative to the oil brake, a generator could also be used.)

In FIG. 3, instead of the oil brake, a second turbine 224 is used, and said turbine is coupled to a generator 261. (As an alternative to the generator, a dissipative brake can also be used, for example an oil brake.) The first turbine 124 is mechanically connected only to the cold secondary compressor 110. The two turbines 124, 224 in each case have the same pressures and temperatures at the inlet and the outlet.

FIG. 4 is thus distinguished from the previous sample embodiments in that the return stream 357 is branched off from the first partial stream even before the cold end of the main heat exchanger, i.e., at a somewhat higher temperature, which does not have to be higher than the dew point, in this example below 130 K. The cold generated in the expansion in the choke (throttle) valve 358 can thus contribute to the cooling of feed air in the main heat exchanger 13.

Contrary to the previous sample embodiments, the pre-cooling stage 4 and the purification stage 5 can be performed at a lower pressure than the first air pressure, as is shown in FIG. 5. In addition to the main air compressor 403, the system must then have another externally-driven secondary air compressor 463, which further compresses all of the purified air 462 from this lower air pressure of, for example, 18 bar, to the first air pressure. (The secondary air compressor 463 has a secondary condenser, which is not depicted in the drawing. It can be made in one or more stages, and it can optionally have an intermediate cooling stage.)

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

The entire disclosures of all applications, patents and publications, cited herein and of corresponding European patent application No. 11009537.9, filed Dec. 1, 2011, are incorporated by reference herein. 

1. A method for producing pressurized oxygen by low-temperature separation of air in a distillation column system for nitrogen-oxygen separation, comprising a low-pressure column (17) and a high-pressure column (18), said method comprising: compressing all of the feed air (1) to a first air pressure (3; 403, 463), which is at least 4 bar over the operating pressure of the high-pressure column (18), cooling at least a portion of the compressed feed air (6) in a main heat exchanger (13), further compressing a first partial stream (7, 9; 107) of the feed air (6), already compressed to said first air pressure, in a secondary compressor (10; 110) to a second air pressure that is higher than said first air pressure, liquefying or pseudo-liquefying the further-compressed first partial stream (12; 112) in said main heat exchanger 13 and then introduced at least a part (15) of the liquefied or pseudo-liquefied first partial stream into said distillation column system for nitrogen-oxygen separation, cooling a second partial stream (8, 23; 123) of the feed air (6), compressed to the first air pressure, in said main heat exchanger (13) to an intermediate temperature and then actively depressurizing the cooled second partial stream in a first expander (24; 124), whereby said first expander (24; 124) drives said secondary compressor (10, 110), introducing the expanded second partial stream (25) into said distillation column system for nitrogen-oxygen separation, withdrawing an oxygen product stream 39 in liquid form from said distillation column system for nitrogen-oxygen separation, and bringining said oxygen product stream 39 in liquid form to an elevated pressure (40), evaporating or pseudo-evaporating and heating under elevated pressure the oxygen product stream 39, in pressurized in liquid form, in said main heat exchanger (13) and recovering said oxygen product stream 39 as a pressurized oxygen product (42), branching off a return stream (57; 357) off from the (pseudo-)liquefied first partial stream (14), depressuring said return stream (57; 357) in a choke valve (58; 358) and heating the depressurized return stream in said main heat exchanger (13), and feeding the heated return stream (59; 159) to said first partial stream (7; 107) at a point upstream from the secondary compressor (10; 110).
 2. The method according to claim 1, wherein compression of the feed air (1) to the first air pressure is performed in a main air compressor (3).
 3. The method according to claim 1, wherein compression of the feed air (1) to the first air pressure is performed in a main air compressor (403) and in a secondary air compressor (463).
 4. The method according to claim 1, wherein the division of the feed air into said first partial stream 107 and said second partial stream 123 occurs within said main heat exchanger
 13. 5. The method according to claim 1, wherein the secondary compressor 110 is operated as a cold compressor, with a starting temperature more than 20K below ambient temperature.
 6. The method according to claim 5, wherein the return stream 159 is combined with the first partial stream 107 only downstream from the cooling of the first partial stream 107, and the division of the feed air into said first partial stream 107 and said second partial stream 123 occurs within said main heat exchanger
 13. 7. The method according to claim 4, wherein said expander 124 is mechanically coupled to said secondary compressor 110, and an oil brake 161 sits on the common shaft of said expander 124 and secondary compressor
 110. 8. The method according to claim 4, wherein a portion of said second partial stream (123) of the feed air (6) is depressurized in a second expander 224 which is coupled to a generator and said first expander (24; 124) mechanically connected only to the said secondary compressor
 110. 9. The method according to claim 1, wherein the return stream 357 is branched off from the first partial stream at a point before the cold end of the main heat exchanger.
 10. The method according to claim 2, wherein the feed air is subjected to a pre-cooling stage 4 and the purification stage 5 after all of the feed air is compressed to said first air pressure.
 11. The method according to claim 3, wherein the feed air is subjected to a pre-cooling stage 4 and the purification stage 5 prior to all of the feed air being compressed up to said first air pressure in the secondary air compressor (463).
 12. An apparatus for producing pressurized oxygen by low-temperature separation comprising: a distillation column system for nitrogen-oxygen separation, said system comprising a low-pressure column (17) and a high-pressure column (18), means for compressing all of the feed air (1) to a first air pressure (3; 403, 463), which is at least 4 bar above the operating pressure of said high-pressure column (18), a main heat exchanger (13) for cooling at least a portion of the compressed feed air (6), a secondary compressor (10; 110) for further compressing a first partial stream (7, 9; 107) of the feed air (6), that is already compressed to the first air pressure, to a second air pressure, which is higher than the first air pressure, means for liquefying or pseudo-liquefying further-compressed first partial stream (12; 112) in said main heat exchanger and means for introducing at least a portion of resultant (pseudo-)liquefied first partial stream into said distillation column system for nitrogen-oxygen separation, means for cooling a second partial stream (8, 23; 123) of the feed air (6). that is already compressed to the first air pressure, to an intermediate temperature in said main heat exchanger (13), an expander (24; 124) for active depressurization of cooled second partial stream, whereby said expander (24; 124) is mechanically coupled to said secondary compressor (10, 110), means for introducing expanded second partial stream (25) into said distillation column system for nitrogen-oxygen separation, means for withdrawing off an oxygen product stream 39 in liquid state from said distillation column system for nitrogen-oxygen separation, means (40) for increasing the pressure of the oxygen product stream 39 in the liquid state to an elevated pressure, means in said main heat exchanger (13) for evaporation or pseudo-evaporation and heating of pressurized liquid oxygen product stream 39 and means for recovering heated pressurized oxygen product stream (42) from said main heat exchanger (13), means for branching off a return stream (57; 357) from the (pseudo-)liquefied first partial stream (14), a choke valve (58; 358) for depressurizing the return stream (57; 357), means for heating the depressurized return stream in said main heat exchanger (13), and means for supplying heated depressurized return stream (59; 159) to the first partial stream (7; 107) upstream from the secondary compressor (10; 110). 